An apparatus of the above described type has already been proposed. With that apparatus, light is conveyed from a so-called wide-band light source through a rotating filter plate containing two filters which transmit light of varying wavelengths. Only one light beam of one wavelength is transmitted by each filter plate. This light beam is deflected by means of a passive reflector onto the measured object from the surface of which the light beam is reflected fan-wise. The fan-shaped reflected light is conducted by means of a concave mirror to a photo-detector for evaluation. The two filters are selected as to their transmissivity to light so that the light of one wavelength is absorbed by the irradiated surface of the measured object, while light of the other wavelength is not absorbed. If, for example, humidity on the surface of a solid object is to be measured, then one filter must pass light of a wavelength which is absorbed by water. The wavelength of the light passed by the other filter must be selected so that this light is not absorbed by water. The result of the measurement can then be deduced from the absorption differential of the two light beams.
The disadvantage of the above considered proposed apparatus is that the parameters of the apparatus are not stable and can result in inaccuracies of measurement. Thus, for example, the filters can, under the influence of temperature, undergo changes with regard to the wavelength with which alone they transmit light. The detector used also exhibits a temperature-dependent behavior.
To overcome the above described difficulties, it is already known (German laid-open application No. 28 16 541) in connection with an apparatus of the type considered above, to derive from the wide-band light source a second light beam which is also conducted through the filter plate, but spaced from and parallel with the light beam which is conducted to the measured object. The second light beam is formed alternatingly by light of one wavelength and light of the other wavelength (corresponding to the wavelengths transmitted by the two filters), like the light beam directed onto the measured object. The arrangement is such that, when the light beam directed onto the measured object is formed by light of one wavelength, the second light beam is formed by light of the other wavelength, and vice versa. In this manner, the detector received successively four different light beams. Although evaluation of these four light beams permits elimination of changes in the parameters of the apparatus, this known apparatus also has the disadvantage of relatively high construction costs as well as the use of a driven filter plate.
Starting from an apparatus of the type just described, it has also been proposed to conduct from the wideband light source a single light beam through the filter plate which is in the form of a filter wheel and which, again, is equipped with two filters which transmit light of different wavelengths. The light beam which is being passed through the filter wheel is then formed alternatingly by light of one wavelength and light of the other wavelength. The light beam leaving the filter wheel is then conducted onto a semitransparent mirror which conducts one part of the light onto the measured object and another part of the light onto a reference detector. The light reflected by the surface of the measured object is conducted by means of a concave mirror onto a measuring detector. Thus, the measuring detector and the reference detector receive, alternatingly, light of variable wavelength. The measuring results of the two detectors serve, on the one hand, to evaluate the variable reflection power of the surface of the measured object determined with the two light beams of different wavelength, and, on the other hand, the measuring results serve to eliminate fluctuations in the parameters of the apparatus. Although with this apparatus, light beams of identical wavelength are used simultaneously for measuring and for reference forming, here also the use of a filter plate is a disadvantage because of the resulting relatively high costs of construction.
The underlying object of the present invention is therefore to show a way how an apparatus of the type indicated in the introduction can be constructed with lesser costs than is the case with the above considered devices.